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	Thu, 20 Jun 2019 14:34:28 -0700 (PDT)
Subject:	Re: malloc() returns pointer to already allocated memory
To:	djgpp AT delorie DOT com
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Date:	Thu, 20 Jun 2019 23:33:12 +0200
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On 2019-06-20 13:21, Rod Pemberton wrote:
> On Thu, 20 Jun 2019 02:22:43 +0200
> "J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]"
> <djgpp AT delorie DOT com> wrote:
> 
>> On 2019-06-19 03:43, Rod Pemberton wrote:
> 
>>> b) how did you notice this issue originally?
>>> c) is the "clobbering" actually causing corruption in your
>>> program?
>>
>> It is causing very obvious corruption, that's how I discovered it.
>> Variables changing for no reason. Then a pointer or some offset
>> changes and the next access triggers a page or GP fault.
> 
> Does DJGPP's symify tell you anything about the page fault or GP fault?

Not really, I can look up the fault locations in a disassembly and then 
manually work out where it occured. Besides I implemented my own 
exception handlers and backtrace so symify wouldn't know how to deal 
with that.

> Does a printf() placed nearby eliminate the issue? (memory allocation)

Doesn't eliminate it, but any code change (including adding printf/cout) 
tends to change the memory location where the corruption occurs.

> Are you accessing memory that hasn't been allocated? (buffer overflow)

As far as I'm aware, no. (and if I was, I would stop doing it :))
For the most part I'm using c++ constructs like std::vector and 
std::unique_ptr which are designed to prevent these sort of issues.

> Are you using any assembly? (register corruption) > Are you using any other "advanced" features of DJGPP like DPMI to
> allocate memory, nearptr's or farptr's, transfer buffer, etc?

There's a lot of that going on, and most of those features I implemented 
myself to be more in line with idiomatic c++ code. However I used those 
same routines in other programs and it doesn't cause any issues there.

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